Re: [PATCH 1/2] mm/page_alloc: prevent MIGRATE_RESERVE pages from being misplaced

[Joonsoo Kim]

On Thu, Apr 03, 2014 at 05:40:17PM +0200, Vlastimil Babka wrote:
> For the MIGRATE_RESERVE pages, it is important they do not get misplaced
> on free_list of other migratetype, otherwise the whole MIGRATE_RESERVE
> pageblock might be changed to other migratetype in try_to_steal_freepages().
> 
> Currently, it is however possible for this to happen when MIGRATE_RESERVE
> page is allocated on pcplist through rmqueue_bulk() as a fallback for other
> desired migratetype, and then later freed back through free_pcppages_bulk()
> without being actually used. This happens because free_pcppages_bulk() uses
> get_freepage_migratetype() to choose the free_list, and rmqueue_bulk() calls
> set_freepage_migratetype() with the *desired* migratetype and not the page's
> original MIGRATE_RESERVE migratetype.
> 
> This patch fixes the problem by moving the call to set_freepage_migratetype()
> from rmqueue_bulk() down to __rmqueue_smallest() and __rmqueue_fallback() where
> the actual page's migratetype (e.g. from which free_list the page is taken
> from) is used. Note that this migratetype might be different from the
> pageblock's migratetype due to freepage stealing decisions. This is OK, as page
> stealing never uses MIGRATE_RESERVE as a fallback, and also takes care to leave
> all MIGRATE_CMA pages on the correct freelist.
> 
> Therefore, as an additional benefit, the call to get_pageblock_migratetype()
> from rmqueue_bulk() when CMA is enabled, can be removed completely. This relies
> on the fact that MIGRATE_CMA pageblocks are created only during system init,
> and the above. The related is_migrate_isolate() check is also unnecessary, as
> memory isolation has other ways to move pages between freelists, and drain
> pcp lists containing pages that should be isolated.
> The buffered_rmqueue() can also benefit from calling get_freepage_migratetype()
> instead of get_pageblock_migratetype().
> 
> A separate patch will add VM_BUG_ON checks for the invariant that for
> MIGRATE_RESERVE and MIGRATE_CMA pageblocks, freepage_migratetype must equal to
> pageblock_migratetype so that these pages always go to the correct free_list.
> 
> Reported-by: Yong-Taek Lee <ytk.lee@xxxxxxxxxxx>
> Reported-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@xxxxxxxxxxx>
> Suggested-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> Suggested-by: Mel Gorman <mgorman@xxxxxxx>
> Cc: Minchan Kim <minchan@xxxxxxxxxx>
> Cc: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
> Cc: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: Rik van Riel <riel@xxxxxxxxxx>
> Cc: Michal Nazarewicz <mina86@xxxxxxxxxx>
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>

Looks good to me.

Acked-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
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